1. Field of the Invention
The present invention relates to a rotation angle detection apparatus used for detecting a rotation angle of a steering part of a vehicle, and the like.
2. Background Art
Recently, rotation angle detection apparatuses for detecting a rotation angle of a steering part that rotates in a linked motion with the rotation of a steering wheel have been becoming widespread. The rotation angle detection apparatus is used for improving the running stability of a vehicle, automatic operation, assisting a driver in parking a vehicle into a garage, and the like.
In such apparatuses, it is very important to detect a rotation angle of a steering part that is rotated when a driver operates a steering wheel. In particular, apparatuses having an accurate detection angle precision and capable of detecting an absolute angle have been demanded.
Such a conventional rotation angle detection apparatus is described with reference to FIGS. 5 and 6.
FIG. 5 is a plan view showing a conventional rotation angle detection apparatus 60. Rotation angle detection apparatus 60 includes main rotatable body 61 having gear 61A on the periphery thereof. In the center of main rotatable body 61, a steering part (not shown) is inserted. Main rotatable body 61 is locked to this steering part.
Rotation angle detection apparatus 60 further includes sub rotatable bodies 62 and 63 engaged with main rotatable body 61. Sub rotatable bodies 62 and 63 respectively have gears 62A and 63A whose numbers of teeth are slightly different from each other. Gears 62A and 63A mesh with gear 61A in different positions of main rotatable body 61. Furthermore, sub rotatable bodies 62 and 63 have angle detection parts 64 and 65, respectively.
Angle detection part 64 includes magnet M1 placed on the upper surface of sub rotatable body 62 and magnetic sensor S1 disposed so as to face the lower surface of sub rotatable body 62. Similarly, angle detection part 65 includes magnet M2 placed on the upper surface of sub rotatable body 63 and magnetic sensor S2 disposed so as to face the lower surface of sub rotatable body 63.
Angle signals transmitted from angle detection parts 64 and 65 are input into a control circuit (not shown). Thus, rotation angle detection apparatus 60 is constructed.
Next, an operation of rotation angle detection apparatus 60 is described.
When a user rotates a steering part, main rotatable body 61 is rotated according to the rotation of the steering part. According to the rotation of main rotatable body 61, sub rotatable bodies 62 and 63 meshing with gear 61A of main rotatable body 61 are also rotated. According to the rotation of sub rotatable bodies 62 and 63, magnets M1 and M2 placed on the upper surface of sub rotatable bodies 62 and 63 are rotated, respectively. Magnetic sensors S1 and S2 for detecting magnetism detect the rotation of magnets M1 and M2, respectively. The control circuit carries out processing of these two detection signals and the numbers of teeth of sub rotatable bodies 62 and 63, and thereby a rotation angle of main rotatable body 61 is detected. Conventionally, a rotation angle of a steering part has been detected in this way.
Note here that the number of teeth of sub rotatable body 62 is set to be ⅓ of that of main rotatable body 61 and the number of teeth of sub rotatable body 62 is set to be slightly different from that of sub rotatable body 63. Therefore, as shown in voltage waveform in FIG. 6, the phase of the signal detected by magnetic sensor S1 is slightly different from the phase of the signal detected by magnetic sensor S2.
For example, when main rotatable body 61 rotates by rotation angle X1, voltage V1 of magnetic sensor S1 and voltage V2 of magnetic sensor S2 are input into the control circuit. The control circuit carries out processing of the two voltage values V1 and V2 and the numbers of teeth of sub rotatable bodies 62 and 63, and thereby rotation angle X1 is detected.
A prior art technology relating to the invention of this application is disclosed in, for example, International Publication WO 99/12796.
However, in the above-mentioned conventional rotation angle detection apparatus 60, the detection of a rotation angle by the control circuit requires complicated processing. Furthermore, when there is a difference in sensitivity between magnetic sensor S1 and magnetic sensor S2, since the difference between peak values of S1 and S2 shown in FIG. 6 occurs, error in the measured angle may occur. Therefore, the sensitivities of two magnetic sensors S1 and S2 have to be matched to each other precisely.